Side-lobe characteristics which are required for antennas used in radio systems, such as point-to-point, are specified in international standards, and it is necessary to suppress the side lobe level to be lower than a predetermined level. Typical international standards are ETSI (European Telecommunications Standards Institute) standards.
A parabola antenna is generally used as an antenna for point-to-point communication. However, when the parabola antenna satisfies the side-lobe standards, the thickness of the antenna increases, which results in an increase in the size of the entire apparatus. For this reason, a planar antenna is desired.
In a millimeter wave band, a planar antenna including a waveguide with a transmission loss lower than that of a microstrip line is used. As a configuration of such a planar antenna, a configuration in which horn antennas are arranged in an array is known (Patent Literature 1). Patent Literature 1 proposes a planar antenna in which horn antennas are arranged in a square lattice. This antenna is characterized by including a box horn at which each horn antenna has a step-like change in shape.